Disclosed embodiments of the present invention relate to circuits, and more particularly, to radio frequency communication circuits.
In a direct conversion radio receiver or transceiver, an RF (Radio Frequency) signal is directly down-converted to a baseband signal without first down-converting to an IF (Intermediate Frequency) signal. For many types of receivers, both the inphase and quadrature components of the baseband signal are employed for further demodulation or detection. A simplified functional diagram of a transceiver employing direct conversion is illustrated in FIG. 1.
In FIG. 1, antenna 102 receives an RF signal. When the transceiver is in its receive mode, the received RF signal is filtered by bandpass filter 104 and mixed with quadrature RF local oscillator signals cos(xcfx89ct) and sin(xcfx89ct) by mixers 106 and 108, respectively, where xcfx89c is the carrier frequency in radians/sec. The output signals of mixers 106 and 108 are filtered by low pass filters 110 and 112, respectively, to provide the inphase (I) and quadrature (Q) components. Detector 114 further processes the I and Q components to provide the final digital data to the end user. Detector 114 may employ matched filtering, error detection and correction, spread-spectrum filtering (de-spreading), and other forms of detection appropriate for the particular modulation and coding scheme that is employed. When the transceiver is in its transmit mode, digital data provided by the end user is encoded by encoder 116, modulated to a baseband signal by modulator 118, up-converted via mixer 120 by mixing with cos(xcfx89ct) and band pass filtered by filter 122. The quadrature RF local oscillator signals cos(xcfx89ct) and sin(xcfx89ct) may be obtained from VCO (Voltage Controlled Oscillator) 126 within PLL (Phase Lock Loop) 124.
If the oscillation frequency of PLL 124 is substantially the same as the carrier frequency, xcfx89c, then VCO 126 may be susceptible to xe2x80x9cfrequency pullingxe2x80x9d during transmission of an RF signal. This may be caused by VCO 126 locking on to the frequency of the transmitted RF signal during transmission because of unintended feedback. If FSK (Frequency Shift Key) modulation is employed, then the transmitted RF signal has a varying instantaneous frequency, possibly centered around xcfx89c. VCO 126 cannot instantly return back to the carrier frequency xcfx89c when the transceiver enters its receive mode, in which case a received RF signal may not be accurately mixed down to its baseband I and Q components.
VCO 126 is less susceptible to frequency pulling if its oscillation frequency is substantially different from xcfx89c. Thus, one approach to mitigating frequency pulling of VCO 126 is to run VCO 126 at twice the carrier frequency, and then synthesize the quadrature mixing signals by a divide-by-two division circuit. This is illustrated in FIG. 2, where VCO 202 oscillates at 2xcfx89c and JK flip flop 204 is configured as a divide-by-two division circuit. One of the quadrature mixing components is taken from output port 206 of JK flip flop 204. The other quadrature mixing component is taken at output port 208 of XOR (exclusive-OR) gate 210.
The quadrature mixing signals in FIG. 2 are square waves, whereas in FIG. 1 the quadrature mixing signals are represented by analog sinusoids, e.g., cos(xcfx89ct) and sin(xcfx89ct). It is to be understood that FIG. 1 is a representation of a communication circuit at a functional level, and that for particular implementations, the quadrature mixing signals may be square waves of frequency xcfx89c rather than analog sinusoids of frequency xcfx89c. It is to be understood throughout these letters patent that sinusoids may be interpreted as square waves, or other discrete-valued periodic waveforms.
As carrier frequencies are increased for communication circuits, running a VCO at twice the carrier frequency may add unwanted cost. For example, in a communication circuit for the consumer market, such as a transceiver circuit in a cordless phone, the carrier frequency may be on the order of 2.5 GHz, so that running a VCO at 5 GHz may add to its manufacturing cost.